High frequency signal transmission system



April 29, 1941. w. s. PERclvAL -2,239,901

HIGH FEQUENCY SIGNAL TRANSMISSION SYSTEM mea sept. 25, 193s f2 -I- g n T T INVENTOR wlLLlAs. PERCIVAL TTORNEY Patented pr. 29, 1941 HIGH FREQUENCY SIGNAL TRANSBIISSION SYSTEM William Spencer Percival, London, England, assignor to Electrical & Musical Industries Limited, Middlesex, England, a company of Great Britain Application September 25, 1936, Serial No. 102,485 In Great Britain November 8, 1935 9 Claims.

The present invention relates lto electric signal transmission systems, and is particularly concerned with the reduction of transient interference in such systems, due to disturbing signals. The expression transient interference is intended to include any interference other than a modulated cr unmodulated continuous wave.

Considerable interference is experienced, more particularly in Wireless and other carrier wave transmission systems, due to disturbing signals of a transient nature, such as are produced, for example, by the ignition systems of internal combustion engines; these disturbing signals are often of very great amplitude, and are believed to consist of short trains of heavily damped oscillations with a natural frequency of the order, approximately, of 40 megacycles per second, each train lasting about 1 micro-second or less. It is believed that the decay period of such a train, that is, the time taken for the amplitude to decay to (that is, about 0.368) of its initial value, is about 0.05 micro-second, corresponding to about 2 or 3 complete cycles of oscillation, and, on account of this high damping, such a disturbing signal may cause interference within a Wide frequency band,

t will be clear that ii such a disturbing signal is app-lied to a sharply-tuned, lightly-damped circuit, tlie time for which the resulting interference lasts may be many times the duration of the signal itself, due to the lightly-damped circuit oscillating at its own natural frequency under the iniiuence of the disturbing signal. By way of example, if a transient disturbing signal having an amplitude p times the maximum amplitude of a desired signal is applied, together with the desired signal, to a tuned circuit, the disturbing signal will require a period 2 (1+10s ev) seconds to decay to an amplitude which is times the maximum amplitude oi the desired signal, Where f is the cut-ofi` frequency of the tuned circuit measured from the resonant frequency; thus for a single tuned circuit cutting off at 1.5 megacycles, the value of t when p is 20, for example, is 0.48 micro-second, while if f is 10 kilocycles7 and p is again 20, t is 72 micro-seconds. It is clear from these examples that since transient disturbing signals are found to cause marked interference in sound transmission systems, such signals must be of very large amplitude; in picture transmission systems, any disturbing signal which produces a signal at the picture reproducer corresponding to a picture brightness value or in the Whiter-than-White region may produce inter-v ference, since persistence of vision may cause the eiect of the disturbing signal to be prolonged.

Other disturbing signals, such as those due to atmospherics, are also known to be of a transient nature, and may also have a very large amplitude..

It is the principal object of the present invention to provide means capable of reducing the efrect of this and other kinds of transient interference, whether the amplitude of the interference is greater or less than that of the desired signal.

It is a further object of the present invention to provide improved means responsive to transient interference of amplitude greater than that of the desired signal for reducing the effect of such interference.

According to the present invention there is provided a circuit arrangement comprising a signal receiving circuit which is subject to transient interference both of greater and less amplitude than that of the maximum desired signal and means responsive to said transient interference for reducing the amplitude of the received signals in said circuit during the presence therein of said transient interference.

According to a feature of the present invention, a circuit arrangement for reducing or removing the effect of transient interference from received signals comprises a main signal channel with means :for applying thereto received signals which are subject to transient interference, an auxiliary channel with means for applying thereto the same transient interference and means for utilising the transient interference in the auxiliary channel to reduce substantially to zero the ampliiication of the main channel in the presence therein of said transient interference. The auxiliary channel may be provided with a limiting device arranged to pass only signals exceeding a predetermined amplitude, which is usually that or the maximum desired signal, the signals so passed being utilised to reduce substantially to zero the amplication of the main channel.

The present invention also provides a circuitarrangement orthe reception of modulated carrier oscillations which comprises a main signal channel of relatively high selectivity with means for applying thereto received signals Which are subject to transient interference, an auxiliary channel of relatively loW selectivity with means for applying thereto the same transient interference and means for utilising the transient interference in the auxiliary channel to reduce the amplification or increase the attenuation of the main channel in the presence therein of said transient interference.

The invention will be described by way of example with reference to the accompanying drawing illustrating one circuit embodying the present invention.

Terminal I1 is the input terminal to a main channel and terminal Iz is the input terminal to an auxiliary channel. These terminals are connected to the same or different receiviny aerials. The main channel is relatively sharply tuned to the frequency of the carrier it is desired to receive, that is to say it has a relatively narrow pass band, While the auxiliary channel is relatively atly tuned.

The auxiliary channel comprises a circuit LiCi tuned to the frequency of the desired carrier or to some other frequency. A valve V1 acts as a mixer valve for mixing the oscillations iror. circuit LiCi with local oscillations from an o.,- cillator V2. In. one example circuit LiCi is tuned to a frequency between 15 and 45 megacycles per second and the oscillatorl V2 serves to produce. an intermediate frequency oi 4.3 megacycles er second. The valves V3, V4, V5 and Vo are arranged to amplify at this intermediate frequency. Tuning of the intermediate frequency stages is effected with the aid of variable inductances Li and L5 which are damped by means of resistances R1 and R2. The tuning of the various stages may be staggered so that with the damping resistances the desired breadth of tuning is obtained. The pass-band for the intermediate frequency ampliner may bc 150 kilocyclcs pe' second.

The output of the intermediate frequency amis coupled to a diode detector V7 the output of which passes through a filter to the inner grid of a heptode valve Va which constitutes the Ilrst valve of the main channel. The nlter comprises shunt condensers C5, Ce and C7 and series elements, each comprising.,r a resistance and an inductance, RsLv, RsLa and Pte-La. The lter is so designed as to remove the carrier frequency of 4.53 megacycles per second and to pass Without substantial attenuation the modulation frequencies passed by the intermediate frequency amplifier, that is frequencies below about 75 kilocycles per second. It is also designed so as to avoid resonances which might cause transients to give rise to variations of an oscillatory character on; the inner grid of valve Vs. The output of valve Va is taken from terminals P to a receiver of normal type which with the valve Ve constitutes the main channel and passes a narrower band of frequencies than the intermediate irequency amplifier V3, V4, V5 and VG.

A suitable positive bias is applied to the cathode of valve Va relative to its control grids by means of a resistance Re shunted by a by-pass condenser C3. A suitable voltage may be impressed between terminals V to make the cathode of the diode V7 somewhat positive relatively to the anode thereof and thus to delay the diode and prevent it passing current for signals below a predetermined amplitude.

The input terminal Il oi the main channel is connected to the outer control grid of the valve Vs 'through a band-pass delay network L1QC3 LUC; which is designed to give to the signals passed `therethrough a delay equal to or slightly greater than the delay introduced in signals passing from terminal I2 to the valve Va.

The operation of the circuit above described is as follows:

It Will rst be assumed that, Whether the input I2 is connected to the same aerial as or to a diiierent aerial from the input I1, the circuit LiCi is uned to a frequency dii'l'erent from that to which the main channel is tuned and that the frequency chosn for circuit LiCi is such that no carrier or substantial strength is eceived Both terminals Ii and I2, however, receive the same transient interference and that in the auxiliary channel is detected at V7, if the transient is above the mi iirnum amplitude (determined by the voltage at V) to which 'the detector V1 is responsive, and is applied es a negative pulse to the inner control grid of valve Vs. It is arby a suitable choice oi valve and bias ier Vs that even a small transient interference is capable oi cutting off the valve Va by arresting the flow of electrons therein. Since the delay in the main channel is equal to or slightly greater than that in the auxiliary channel, the ampliiication of valve Vg and hence that or' the main channel, will have been reduced substantially to zero by the time a transient reaches the outer control grid oi valve Va from terminal I1.

t is also preferable to arrange that the condenser C5 with the resistance in parallel therewith has a sufficiently long time constant to maintain the valve Vs cut-oil' lor a suitable short period after the transient has ceased at the input to diode V7. In this way the necessity for a precise adjustment of the delay in the main and auxiliary channels which may be dililcult especially when the tvvo channels are operated at different carrier frequencies, can be avoided.

The voltage from the filter in the output of valve V': may of course be used to cut-ori valves of the main amplifier other than or additional to Vs.

It has been found that with certain types ol transient interference, such as that due to switching, a connection such as that shown, capable of passing direct-l current between the valves Vv and V8 is essential to obtain the best results. However, with other iorms oi' transient interference such as that arising from the ignition systems of motor cars which is oi short duration, this is not the case and the output from the detector Vv may then conveniently be ampliiied before applying it to cut-off the valve Ve. In this case any delaying which may be required can be done with advantage in the added amplier instead ci in the detector V7.

By making the voltage V very small o` aero it can be arranged that transient interference even of very small amplitude serves to cut-ofi the valve V8.

With the arrangement so iar described, it has been assumed that no carrier of substantial amplitude is present in the input to the auxiliary channel and the circuit described can then be used to reduce, if desired substantially to Zero, transient interference appearing in the main channel. The circuit can be made responsive to transients which in the main channel are oi amplitude less than that of the desired carrier well as to transients or greater' amplitude.

Even Where the input to the auxiliary channel is tuned to the same frequency as the main channel, the apparatus described is capable of cutting off the valve Vs in response to transients which in the main channel have an amplitude somewhat less than that of the maximum desired signal, as Well as in response to larger transients. This is because owing to the flatter tuning of the auxiliary channel the ratio of transient amplitude to desired signal amplitude is greater in the auxiliary channel than in the main channel. In the main channel the Vtransient will tend to persist longer than in the auxiliary channel but by a suitable choice of the condenser C and its parallel resistance the valve Va can be maintained cut-off until the transient in the main channel has substantially died away.

However, in order to be able to eliminate from the main channel transient interference of substantially smaller amplitude than the maximum desired signal, it is necessary to arrange that the auxiliary channel does not pass the desired signal, or any other continuous wave, at any substantial strength.

As already stated, the diode Vv may be delayed so as to respond only to signals above a predetermined amplitude. It is of course desirable to make this predetermined amplitude as low as possible provided that it exceeds that of any continuous wave which may appear at the diode V7. According to a feature of the present invention therefore, there is provided a circuit whereby the valve V7 may be set to be responsive to a relatively low amplitude and nevertheless it remains unresponsive to continuous Waves (whether they be of the same frequency as or a different frequency from that of the desired signal), even though the amplitude of these continuous waves may change due to fading for example.

For this purpose a special form of automatic volume control circuit is provided. This comprises an additional intermediate frequency stage including a valve V9 and a tuned circuit L12R5 connected to the output of valve V9. The valve V9 is of such type that when given a low screengrid voltage it operates as a limiter, partly by grid current damping and partly by anode current cut-off. The output of this valve V9 is fed to a diode detector Vio and the output of the latter is fed through a filter F to the grids of one or more of the valves of the intermediate frequency amplier V3, V4, V5, Ve.

The valve V9 ensures that only signals of relatively low amplitude have any effect upon the A. V. C. voltage developed.A Since the energy content of transients of the same amplitude as a continuous wave is much smaller than the energy content of the continuous wave, transients of the same amplitude as the continuous wave have but little inuence on the A. V. C. voltage developed. Thus the A.V V. C. voltage is mainly dependent upon the continuous waves and it can be arranged that the amplication of the intermediate frequency amplier V3, V4, V5, Ve varies as the amplitude of continuous waves varies in such a way as to maintain the output of continuous waves from the valve V6 substantially constant and that the action is substantially unaffected even by transients of very large amplitude.

It may often be advantageous to associate the two channels with diiferentaerials. For exam- -ple the aerial supplying the main channel may be arranged so as to be more sensitive than the other aerial to the desired signal. The latter aerial on the other hand may be made relatively more sensitive to transient interference by suitable location or orientation thereof. For example, the aerial of the auxiliary channel may be a length of wire taken close to a source of interference, the eiect of which on the main ychannel it is required to eliminate.

The two channels may if desired both be arranged to convey useful intelligence. For instance the main, sharply tuned channel may form part of a wireless receiver for a carrier modulated with sound signals whilst the broader auxiliary channel may form part of a television receiver for reproducing pictures to which the sounds are an accompaniment. A detector separate from V7 may be provided to rectify the output of valve Vs and pass the rectified signals to a television reproducer. Y

The circuit Lin, Cs--L11, C4 may be preceded if desired by an ampliiier having a broader pass band than the rest of the main channel followed by an amplitude limiter set to pass only signals having an amplitude equal to or less than that of the maximum desired signal. In this way the tendency for shock excitation of the sharply tuned circuits of the main channel amplier is reduced because only transients of relatively small amplitude reach the sharply tuned circuits.

It is not necessary that the auxiliary channel should have a continuous pass band, nor is it necessary that it should pass equally al1 freduencies within its pass band. For example one or more bands of continuous wave interference or the desired signal band may be substantially suppressed in the auxiliary channel with the aid of one or more suitable band-stop lters or the like.

rThe invention is not confined to circuits dealing with modulatedrcarrier oscillations. For example, the main channel may be constituted by a telephone line carrying speech currents and the auxiliary channel may be a line which is subject to the same interference signals as the main channel but which does not convey thespeech currents.

'The output of the auxiliary channel may be used to switch oit any desired number of main channels.

'I'he result of the application oi the present invention is to produce gaps (or regions of zero amplitude) in the desired signal in place of transient interference. In many cases, particularly where the interference is of very short duration, the gaps may not introduce any appreciable efi'ect. In any case they are greatly to be preferred to transients of amplitude which may in practice reach values many times that of the maximum desired signal.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

l. In combination with a signal receiver having at least one tube provided with a tuned signal input circuit, a control network adapted to r'educe the signal transmission efficiency of said receiver during the reception of transient interference, said control network comprising a rectifier having a resonant input circuit tuned to a radio frequency, means for impressing on said rectiiier input circuit transient interference impressed on said signal input circuit, means responsive to the rectifier output for reducing the gain of said receiver tube, said signal input circuit being of relatively high selectivity, and said rectier input circuit having a relatively low selectivity.

2. In combination with a signal receiver having at least one tube provided with a tuned signal input circuit, a control network adapted to reduce the signal transmission efficiency of said receiver during the reception of transient interference, said control network comprising a rectiiier having a resonant input circuit tuned to a radio frequency, means for impressing on said rectifier input circuit transient interference impressed on said signal input circuit, means responsive to the rectifier output for reducing the gain of said receiver tube, said signal input circuit being of relatively high selectivity, said rectifier input circuit having a relatively low selectivity, said rectifier input circuit comprising a plurality of cascaded, broadly tuned circuits, and said signal input circuit having its constants chosen to delay the signal transmission to an extent at least as great as the delay experienced by signals in the rectifier input circuit.

3. In combination with a signal receiver having at least one tube provided with a tuned signal input circuit, a control network adapted to reduce the signal transmission efciency of said receiver during the reception of transient interference, said control network comprising a rectifier having a resonant input circuit tuned to a radio frequency, means for impressing on said rectifier input circuit transient interference impressed on said signal input circuit, means responsive to the rectifier output for reducing the gain of said receiver tube, said signal input circuit being of relatively high selectivity, and said rectifier input circuit having a relatively low selectivity, said responsive means including a network having a relatively long time constant whereby the duration of the rectifier output applied to the receiver tube is made greater than that of the transient interference applied to the rectifier.

4. In combination with a signal receiver having at least one tube provided with a tuned signal input circuit, a control network adapted to reduce the signal transmission efficiency of said receiver during the reception of transient interference, said control network comprising a rectifier having a resonant input circuit tuned to a radio frequency, means for impressing on said rectifier input circuit transient interference impressed on said signal input circuit, means responsive to the rectifier output for reducing the gain of said receiver tube, said signal input circuit being of relatively high selectivity, said rectifier input circuit having a relatively low selectivity, and means responsive to variations in continuous wave amplitude in said control network for controlling the continuous Wave trans- A',

input circuit transient interference impressed on said signal input circuit, means responsive to the rectifier output for reducing the gain of said receiver tube, said signal input circuit being of relatively high selectivity, and said rectifier input circuit having a relatively low selectivity, said receiver tube being of the type having a signal grid and special gain control grid, and the said responsive means impressing said rectifier output on said special grid in a sense to cause said gain reduction.

6. In combination with a signal receiver having at least one tube provided with a tuned signal input circuit, a control network adapted to reduce the signal transmission efficiency of said receiver during the reception of transient interference, said control network comprising a rectifier having a resonant input circuit tuned to a radio frequency, means for impressing on said rectifier input circuit transient interference impressed on said signal input circuit, means responsive to the rectifier output for reducing the gain of said receiver tube, said signal input circuit being of relatively high selectivity, said rectifier input circuit having a relatively low selectivity, said rectifier input circuit including a converter network constructed to reduce the frequency of continuous waves impressed thereon. an amplifier for the reduced frequency energy, and an automatic gain control circuit for maintaining the amplitude of the amplified energy at the rectifier substantially constant.

7. In combination with a signal receiver having at least one tube provided with a tuned signal input circuit, a control network adapted to reduce the signal transmission efiiciency of said receiver during the reception of transient interference, said control network comprising a rectifier having a resonant input circuit tuned to a radio frequency, means for impressing on said rectifier input circuit transient interference impressed on said signal input circuit, means responsive to the rectifier output for reducing the gain of said receiver tube, said signal input circuit being of relatively high selectivity, said rectifier input circuit having a relatively low selectivity, said rectifier input circuit including at least one broadly tuned amplifier, and means responsive to waves impressed on the control network for controlling the amplifier gain.

S. ln a radio receiver of the type including a tube having at least a cathode, signal grid, output electrode and a gain control electrode, a signal input circuit connected to the signal grid and cathode, an output circuit connected to said output electrode, a noise control network comprising a rectifier, an amplifier having an input circuit tuned to said signal frequency and adapted to have combined signal and noise energy impressed thereon, said amplifier being coupled to the rectifier, means, responsive to the said energy intensity at the rectifier input, for varying the amplifier gain, means for developing potential pulses from the rectified noise energy, and means for impressing the pulses on said gain control electrode in a sense to reduce the receiver tube gain substantially to Zero.

9. In a radio receiver of' the type including a tube having at least a cathode, signal grid, output ele-ctrode and a gain control electrode, a signal input circuit connected to the signal grid and cathode, an output circuit connected to said output electrode, a noise control network comprising a rectifier, an amplifier having an input circuit tuned to said signal frequency and adapted to have combined signal and noise energy impressed thereonI said amplifier being coupled to the rectifier, means for developing potential pulses from the rectified noise energy, means for impressing the pulses on said gain control electrode in a sense to reduce the receiver tube gain substantially to Zero, said amplifier input circuit being substantially less selective than said receiver tube input circuit, and said rectifier being biased to delay rectification of noise energy until the latter increases above the predetermined amplitude.

WILLIAM SPENCER PERCEVAL. 

